[0001] The present invention relates generally to data demodulation in a radio communication
system, and in particular, to an apparatus and method for demodulating data against
signal distortion caused by fading or other factors.
[0002] Radio communication technology, mainly cellular communication technology has been
rapidly developed and GMPCS (Global Mobile Personal Communication System) is being
deployed for communication throughout the world. For radio communication systems utilizing
satellites, research has actively been conducted on data demodulation because the
long distance between a satellite and a mobile station causes severe data distortion
due to fading.
[0003] In general, predetermined symbols (e.g., pilot symbols) are inserted in one frame
prior to transmission and the distortion of other information symbols are compensated
for by detecting the distortion of the predetermined symbols. That is, a transmitter
inserts agreed symbols between data symbols prior to transmission and a receiver extracts
those agreed symbols for use in channel estimation. Conventional channel estimation
relies on the use of an interpolator or extended symbol-aided estimation (ESAE).
[0004] With respect to prior art systems, FIG. 1 is a schematic block diagram of a conventional
channel estimating apparatus in a radio communication system for data recovery. FIG.
2 is the format of a frame used in the conventional radio communication system. FIG.
3 is a detailed block diagram of the conventional channel estimating apparatus in
the radio communication system. FIG. 4 is a block diagram of another conventional
channel estimating apparatus relying on ESAE in a radio communication system and FIG.
5 conceptually illustrates channel estimation for data recovery relying on the ESAE.
[0005] FIG. 1 illustrates a channel estimating apparatus using pilot symbols for channel
estimation in a PSAM (Pilot Symbol Assisted Modulation) system. In the PSAM system,
pilot symbols are periodically inserted by pilot symbol inserter 101 between data
symbols and the entire signal is pulse-shaped by pulse shaper 102 prior to transmission.
Fading and AWGN (Additive White Gaussian Noise) are added to the transmission signal
by multiplier 103 and adder 104, respectively. A receiver separates an input signal
into pilot symbols and data symbols by passing the input signal through a matched
filter 105 and estimates the channel of data symbols using the pilot symbols. For
the channel estimation, an interpolator 108 is required and data is recovered using
the interpolation result. Delay 106 compensates for the signal delay through interpolator
108.
[0006] The channel estimation process in a general PSAM system can be expressed briefly
as


where St(t) is the transmitter signal, Re[zO(t)exp(j2πfct)] represents the real number
part of zO(t)exp(j2πfct), fc is a carrier frequency, and zO(t) is a transmission baseband
signal with its band limited by a transmission filter. As shown in FIG. 2, preset
pilot symbols are inserted into a transmission frame. Due to fading, the transmission
signal arrives at the receiver as

where Sr(t) is the received signal, and nc(t) is the AWGN component. A channel complex
gain c(t) includes fading and a frequency offset, given by

where fo is a residual frequency offset and g(t) is the envelope of c(t). Then, a
demodulated baseband signal is expressed as

[0007] It is necessary to estimate C(t) to achieve the baseband signal Z(t). The sampled
value of an m
th symbol in a k
th frame is

for k = 0, 1, 2, 3, ... n = 0, 1, 2, 3, ..., M-1
where TP, a pilot symbol insertion period, is NT. A pilot symbol demodulated at every
frame timing instant is

[0008] An estimated value of fading at the instant when a k
th pilot symbol is received is computed by dividing a distorted symbol U(tk, 0) of Eq.
(7) by a pilot symbol b. That is,

[0009] Fading-caused distortion of an information symbol can be detected using an interpolator
as applied to Eq. (8). There are generally two interpolation methods: fixed interpolation
and adaptive interpolation. For fixed interpolation, a sync (Nyquist), Gaussian, linear,
or a cubic interpolator is applied throughout a channel to estimate the distortion
of the channel regardless of channel variation, whereas for adaptive interpolation,
for example, a Wiener interpolator using a Wiener filter accurately estimates a channel
by adaptively compensating for channel variation utilizing parameters like Doppler
frequency and symbol energy per power spectrum density (Es/No).
[0010] FIG. 3 is a conceptual view of the fading estimation and compensation using a sync
interpolator. As shown in FIG. 3, for channel estimation, a fading estimator 301 estimates
fading of pilot symbols and an interpolator 302 interpolates data symbols based on
the channel estimation of the pilot symbols. The channel estimation result is reflected
in an input signal delayed by a delay 304, to thereby compensate the input signal.
[0011] FIGs. 4 and 5 illustrate the other channel estimation scheme, ESAE. A receiver separates
an input signal into pilot symbols and data symbols by passing the input signal through
a matched filter 401. For the channel estimation, an interpolator 403 is required
and data is recovered using the interpolation result. First delay 402 compensates
for the signal delay through interpolator 403. Demodulator 405 demodulates the signal.
As shown in FIG. 5, recovered data before a symbol "S" is used along with pilot symbols
"P1", "P2", "P3", and "P4" to estimate the channel of the symbol "S".
[0012] Despite relative simplicity in channel estimation, the data estimation scheme using
pilot symbol channel estimation and the ESAE scheme have shortcomings in that channel
estimation is not reliable when a received signal has weak strength or experiences
severe fading.
[0013] It is, therefore, the object of the present invention to provide a channel estimating
apparatus and method capable of channel estimation even in an environment where fading
causes severe distortion.
[0014] The above object can be achieved by providing a channel estimating apparatus and
method in a radio communication system. In the channel estimating apparatus, a fading
estimator estimates a channel using preset symbols of an input signal, a first interpolator
interpolates the other symbols of the input signal based on the fading estimation,
a first inverter inverts the output signal of the first interpolator, a first delay
delays the input signal for a predetermined time, a first multiplier primarily compensates
the output signal of the first delay by means of the output signal of the first inverter,
a second interpolator interpolates each symbol of the input signal relating to primarily
compensated symbols in a predetermined period before and after the symbol, a level
controller controls the level of the output signal of the second interpolator, a second
inverter inverts the output signal of the level controller, a second delay delays
the primarily compensated signal for a predetermined time, and a second multiplier
secondarily compensates the output signal of the second delay by means of the output
signal of the second inverter.
[0015] The above object, features and advantages of the present invention will become more
apparent from the following detailed description when taken in conjunction with the
accompanying drawings in which:
FIG. 1 is a schematic block diagram of a conventional channel estimating apparatus
in a radio communication system;
FIG. 2 is the format of a frame used in the conventional radio communication system;
FIG. 3 is a detailed block diagram of the channel estimating apparatus in the conventional
radio communication system;
FIG. 4 is a block diagram of another conventional channel estimating apparatus relying
on ESAE in a radio communication system;
FIG. 5 is a conceptual view of channel estimation for data recovery relaying on the
ESAE in the radio communication system;
FIG. 6 is a block diagram of a channel estimating apparatus according to an embodiment
of the present invention;
FIG. 7 is a conceptual view of channel estimation for data recovery according to the
embodiment of the present invention; and
FIGs. 8A to 8D are graphs showing BER characteristics according to the embodiment
of the present invention.
[0016] A preferred embodiment of the present invention will be described herein below with
reference to the accompanying drawings. In the following description, well-known functions
or constructions are not described in detail since they would obscure the invention
in unnecessary detail.
[0017] FIG. 6 is a block diagram of a channel estimating apparatus according to an embodiment
of the present invention and FIG. 7 is a conceptual view of channel estimation for
data recovery according to the embodiment of the present invention.
[0018] Referring to FIG. 6, a fading estimator 600 estimates fading using preset pilot symbols
in an input signal. A first interpolator 610 interpolates information symbols based
on the fading estimation. A Wiener interpolator may be used as the first interpolator
610. A first inverter 620 obtains the reciprocal number of the output of the first
interpolator 610 through inverting. A first delay 630 delays an input signal for a
predetermined time to provide action time to the fading estimator 600, the first interpolator
610, and the first inverter 620. A multiplier 640 primarily compensates the delayed
signal received from the first delay 630 using the channel estimation signal inverted
by the first inverter 620. The primary compensated signal is fed to a second interpolator
650 and a second delay 680. The second interpolator 650 is preferably a sync or Nyquist
interpolator and estimates fading relating to the primarily compensated data as shown
in FIG. 7.
[0019] In FIG. 7, the second interpolator 650 estimates fading relating to the primarily
compensated symbols (marked with slash lines) preceding and following a data symbol
S to be estimated. Pilot symbols P1, P2, P3, and P4 that were used for the primary
compensation may be used along with the primarily compensated symbols for the secondary
channel estimation. The same weight or different weights can be given to the primarily
compensated data symbols and the pilot symbols.
[0020] Referring again to FIG. 6, a level controller 660 controls the level of the estimated
value. For example, if reference symbols are (1, 0) and (-1, 0), all symbols are shifted
to a (1, 0) domain by generalizing the other quadrature phase-shift keying (QPSK)
symbols (0, 1) and (0, -1), for achieving the channel estimated value. A second inverter
670 inverts the level-controlled signal and a multiplier 690 secondarily compensates
the delayed signal received from the second delay 680 by multiplying the delayed signal
by the inverted signal received from the second inverter 670.
[0021] In accordance with the embodiment of the present invention, the channel of the symbol
S is estimated through the primary estimation using pilot symbols and the secondary
estimation using symbols compensated for by the Wiener interpolator 610 as new pilot
symbols using the Nyquist interpolator 650.
[0022] While the Wiener interpolator 610 is used for the primary channel estimation, the
Nyquist interpolator 650 is used for the secondary channel estimation because the
primarily compensated symbols related thereto are located near the symbol to be estimated.
[0023] According to the channel estimation method in the embodiment of the present invention,
data symbols are more accurately channel-estimated as signal to noise ratio (SNR)
increases and use of compensated data symbols along with the pilot symbols increases
channel estimation reliability.
[0024] FIGs. 8A to 8D illustrate BER characteristics according to the embodiment of the
present invention. The horizontal axis of each of FIGs. 8A to 8D represents Es/No
and the vertical axis represents BER variations. Table 1 lists experimental data.
TABLE 1
Figure |
K |
fd |
8A |
7dB |
20Hz |
8B |
7dB |
200Hz |
8C |
12dB |
20Hz |
8D |
12dB |
200Hz |
[0025] As noted from FIGs. 8A to 8D, performance is improved in the embodiment of the present
invention, as compared to performance in a conventional Wiener interpolator using
scheme.
[0026] In Table 1, K denotes a Ricean fading factor and fd is a Doppler frequency. Normalized
Doppler frequencies (fdT) are 0.0011 and 0.011, that is, 20Hz and 200Hz in the case
where a symbol rate is 18,000 symbols per second. Here, a pilot insertion period M
is 20. In the case of coherent demodulation, it is assumed that a receiver accurately
knows required Doppler frequency and symbol energy per power spectrum density (γ =
Es/No). It is concluded from simulation results that the embodiment of the present
invention is superior to the conventional Wiener interpolator using scheme in case
of fast fading (fdT = 0.011), as shown in FIGs. 8B and 8D.
[0027] According to the present invention as described above, a channel is primarily estimated
using pilot symbols and secondarily estimated using the primarily compensated symbols.
Therefore, channel estimation can be performed even at severe distortion caused by
fading.
1. A channel estimating apparatus in a radio communication system, comprising:
a fading estimator for estimating a channel using preset symbols of an input signal;
a first interpolator for interpolating other symbols of the input signal based on
the fading estimation;
a first inverter for inverting an output signal of the first interpolator;
a first delay for delaying the input signal for a predetermined time;
a first multiplier for primarily compensating an output signal of the first delay
by means of the output signal of the first inverter;
a second interpolator for interpolating each symbol of the input signal relating to
primarily compensated symbols in a predetermined period before and after the symbol;
a level controller for controlling the level of an output signal of the second interpolator;
a second inverter for inverting an output signal of the level controller;
a second delay for delaying the primarily compensated signal for a predetermined time;
and
a second multiplier for secondarily compensating an output signal of the second delay
by means of the output signal of the second inverter.
2. The channel estimating apparatus of claim 1, wherein the first interpolator is a Wiener
interpolator.
3. The channel estimating apparatus of claim 1 or 2, wherein the second interpolator
is one of a sync interpolator and a Nyquist interpolator.
4. The channel estimating apparatus of one of claims 1 to 3, wherein the preset symbols
are pilot symbols.
5. A channel estimating method in a radio communication system, comprising the steps
of:
primarily compensating other symbols of an input signal using preset symbols; and
secondarily compensating each symbol of the input signal relating to the primarily
compensated symbols in a predetermined period before and after each symbol.
6. The channel estimating method of claim 5, wherein the primarily compensating step
comprises the steps of:
estimating a channel using the preset symbols;
interpolating the other symbols of the input signal based on the channel estimation
and inverting the interpolated signal; and
compensating the input signal by means of the inverted signal.
7. The channel estimating method of claim 5 or 6, wherein the secondarily compensating
step comprises the steps of:
interpolating each symbol relating to the primarily compensated symbols in the predetermined
period before and after the symbol and controlling the level of the interpolated symbols;
and
inverting the level-controlled signal and secondarily compensating the primarily compensated
signal.
8. The channel estimating method of one of claims 5 to 7, wherein the preset symbols
are pilot symbols.